U.S. patent application number 11/767642 was filed with the patent office on 2008-02-21 for driving method for el displays with improved uniformity.
Invention is credited to Ronald S. Cok.
Application Number | 20080042938 11/767642 |
Document ID | / |
Family ID | 39100928 |
Filed Date | 2008-02-21 |
United States Patent
Application |
20080042938 |
Kind Code |
A1 |
Cok; Ronald S. |
February 21, 2008 |
DRIVING METHOD FOR EL DISPLAYS WITH IMPROVED UNIFORMITY
Abstract
A method of driving an electroluminescent (EL) display having a
plurality of light-emitting display elements having outputs that
change with time or use, comprising the steps of: a) providing an
external image signal with a first image aspect ratio; b) providing
an EL display having light-emitting display elements formed in a
two-dimensional array having a second display aspect ratio
different from the first image aspect ratio; c) driving all of the
two-dimensional array of display elements with a composite signal
comprising the external image signal and an internal aging signal,
wherein a subset of the display elements is driven by the external
image signal and the remainder of the display elements that are not
driven by the external image signal are driven with the internal
aging signal; and d) changing the location of the subset of display
elements within the two-dimensional array driven by the external
image signal over time.
Inventors: |
Cok; Ronald S.; (Rochester,
NY) |
Correspondence
Address: |
David Novais;Patent Legal Staff
Eastman Kodak Company, 343 State Street
Rochester
NY
14650-2201
US
|
Family ID: |
39100928 |
Appl. No.: |
11/767642 |
Filed: |
June 25, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11464688 |
Aug 15, 2006 |
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11767642 |
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Current U.S.
Class: |
345/76 |
Current CPC
Class: |
G09G 3/3216 20130101;
G09G 2320/043 20130101; G09G 2340/0442 20130101; G09G 3/3225
20130101; G09G 2320/029 20130101; G09G 2310/0232 20130101; G09G
3/007 20130101 |
Class at
Publication: |
345/76 |
International
Class: |
G09G 3/30 20060101
G09G003/30 |
Claims
1. A method of driving an electroluminescent (EL) display having a
plurality of light-emitting display elements having outputs that
change with time or use, comprising the steps of: a) providing an
external image signal with a first image aspect ratio; b) providing
an EL display having light-emitting display elements formed in a
two-dimensional array having a second display aspect ratio
different from the first image aspect ratio; c) driving all of the
two-dimensional array of display elements with a composite signal
comprising the external image signal and an internal aging signal,
wherein a subset of the display elements is driven by the external
image signal and the remainder of the display elements that are not
driven by the external image signal are driven with the internal
aging signal; and d) changing the location of the subset of display
elements within the two-dimensional array driven by the external
image signal over time.
2. The method of claim 1, wherein the number of display elements in
the subset of the display elements is smaller than the number of
display elements in each dimension of the two-dimensional array of
display elements.
3. The method of claim 1, wherein the number of light-emitting
elements in the two-dimensional array is larger than the number of
image elements in each corresponding dimension of the external
image signal.
4. The method of claim 1, wherein the internal aging signal has a
luminance value from 10-90% of the average luminance value of the
external image signal over time.
5. The method of claim 1, wherein the internal aging signal has a
luminance value from 25-75% of the average luminance value of the
external image signal over time.
6. The method of claim 1, wherein the internal aging signal has a
luminance value approximately 50% of the average luminance value of
the external image signal over time.
7. The method of claim 1 wherein the internal aging signal depends
on the relative amount of time that the light-emitting elements are
driven with the external image signal or on the average luminance
of the external image signal over time.
8. The method of claim 1 wherein the internal aging signal depends
on the average luminance of a portion of the external image signal
over time.
9. The method of claim 8 wherein the internal aging signal depends
on the average luminance at or near an edge of the external image
signal.
10. The method of claim 8 wherein the internal aging signal depends
on the smaller of the average luminance at or near an edge of the
external image signal and the average luminance of the external
image signal over time.
11. The method of claim 1 wherein the internal aging signal depends
on the average ambient illumination incident on the display.
12. The method of claim 11 wherein the internal aging signal
luminance is increased as the average ambient illumination incident
on the display increases.
13. The method of claim 1 wherein the internal aging signal has a
luminance value of 5% to 20% of the maximum luminance of the
display.
14. The method of claim 1 wherein the internal aging signal has a
luminance value of 10% to 15% of the maximum luminance of the
display.
15. The method of claim 1 wherein the location of the subset of
display elements in the two-dimensional array driven by the
external image signal is changed over time relatively greater in
one dimension than the other.
16. The method of claim 1 wherein the location changes are limited
in a dimension to less than the total number of light-emitting
elements in that dimension.
17. The method of claim 1 wherein the external image signal has an
aspect ratio that is relatively larger than the display aspect
ratio, and the location changes of the subset is greater in the
vertical dimension than in the horizontal dimension.
18. The method of claim 1 wherein the external image signal has an
aspect ratio that is relatively smaller than the display aspect
ratio, and the location changes of the subset is greater in the
horizontal dimension than in the vertical dimension.
19. The method of claim 1 wherein the light-emitting elements form
pixels, each pixel emitting four different colors of light and
having four sub-pixels that each emit one of red, green, blue, and
a broadband light, and the internal aging signal drives both the
broadband light-emitting sub-pixel and the red, green, and blue
sub-pixels at a level corresponding to the relative average usage
of the sub-pixels.
20. An electroluminescent EL display, comprising: a) a plurality of
light-emitting display elements having outputs that change with
time or use, formed in a two-dimensional array having a display
aspect ratio; and b) a controller for receiving an external image
signal with an image aspect ratio different from the display aspect
ratio; for driving all of the two-dimensional array of display
elements with a composite signal comprising the external image
signal and an internal aging signal, wherein a subset of the
display elements is driven by the external image signal and the
remainder of the display elements that are not driven by the
external image signal are driven with the internal aging signal;
and for changing the location of the subset of display elements
within the two-dimensional array driven by the external image
signal over time.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation-in-part of application Ser. No.
11/464,688, filed Aug. 15, 2006 entitled "Driving OLED Display With
Improved Uniformity" by Ronald S. Cok.
FIELD OF THE INVENTION
[0002] The present invention relates to solid-state
electroluminescent flat-panel display devices and more particularly
to methods for driving such display devices to reduce differential
aging of the light-emitting display and provide improved display
uniformity.
BACKGROUND OF THE INVENTION
[0003] Electroluminescent (EL) devices are a promising technology
for flat-panel displays and area illumination lamps. For example,
Organic Light Emitting Diodes (OLEDs) have been known for some
years and have been recently used in commercial display devices. EL
devices rely upon thin-film layers of materials coated upon a
substrate, and include organic, inorganic and hybrid
inorganic-organic light-emitting diodes (LEDs). The thin-film
layers of materials can include, for example: organic materials;
inorganic materials such as quantum dots; fused inorganic
nano-particles; electrodes, for example, made of metals or metal
oxides or alloys thereof; conductors; and silicon or metal oxide
electronic components (e.g., zinc oxide) as are known and taught in
the LED art. Such EL devices employ both active-matrix and
passive-matrix control schemes and can employ a plurality of
light-emitting elements. The light-emitting elements are typically
arranged in two-dimensional arrays with a row and a column address
for each light-emitting element, and are driven by a data value
associated with each light-emitting element to emit light at a
brightness corresponding to the associated data value.
[0004] Typical large-format displays (e.g. having a diagonal of
greater than 12 to 20 inches) employ hydrogenated amorphous silicon
thin-film transistors (aSi-TFTs) formed on a substrate to drive the
pixels in such large-format displays. The manufacturing process
conventionally employed to form aSi-TFTs typically produces TFTs
whose characteristics vary spatially over the surface of the
substrate. However, the local aSi-TFT variation is typically
relatively small so that neighboring TFTs will have similar
characteristics while TFTs spaced further away will vary more.
[0005] Moreover, as described in "Threshold Voltage Instability Of
Amorphous Silicon Thin-Film Transistors Under Constant Current
Stress" by Jahinuzzaman et al. in Applied Physics Letters 87,
023502 (2005), the aSi-TFTs exhibit a metastable shift in threshold
voltage when subjected to prolonged gate bias. This shift is not
significant in traditional display devices such as LCDs, because
the current required to switch the liquid crystals in LCD display
is relatively small. However, for LED applications, much larger
currents must be switched by the aSi-TFT circuits to drive the
electroluminescent materials to emit light. Thus,
electroluminescent displays employing aSi-TFT circuits are expected
to exhibit a significant voltage threshold shift as they are used.
This voltage shift may result in decreased dynamic range and image
artifacts. Moreover, the organic materials in OLED and hybrid EL
devices also deteriorate in relation to the integrated current
density passed through them over time, so that their efficiency
drops while their resistance to current increases.
[0006] One approach to avoiding the problem of voltage threshold
shift in TFT circuits is to employ circuit designs whose
performance is relatively constant in the presence of such voltage
shifts. For example, US2005/0269959 filed by Uchino et al., Dec. 8,
2005, entitled "Pixel Circuit, Active Matrix Apparatus And Display
Apparatus" describes a pixel circuit having a function of
compensating for characteristic variation of an electro-optical
element and threshold voltage variation of a transistor. The pixel
circuit includes an electro-optical element, a holding capacitor,
and five N-channel thin-film transistors including a sampling
transistor, a drive transistor, a switching transistor, and first
and second detection transistors. Alternative circuit designs
employ current-mirror driving circuits that reduce susceptibility
to transistor performance. For example, US2005/0180083 filed by
Takahara et al., Aug. 15, 2005 entitled "Drive Circuit For El
Display Panel" describes such a circuit. However, such circuits are
typically much larger and more complex than the two-transistor,
single capacitor circuits otherwise employed, thereby reducing the
area on a display available for emitting light and decreasing the
display lifetime.
[0007] Other methods useful for aSi-TFTs rely upon reversing or
slowing the threshold-voltage shift. For example, US2004/0001037
filed Jan. 1, 2004 by Tsujimura et al., entitled "Organic
Light-Emitting Diode Display" describes a technique to reduce the
rate of increase in threshold voltage, i.e. degradation, of an
amorphous silicon TFT driving an OLED. However, such schemes
typically require complex additional circuitry, thereby reducing
the geographical area on a display available for emitting light and
decreasing the display lifetime.
[0008] In the case of OLED and hybrid EL devices, as the display is
used the organic materials in the device age and become less
efficient at emitting light. This reduces the lifetime of the
display. The differing organic materials may age at different
rates, causing differential color aging and a display whose white
point varies as the display is used. If some light-emitting
elements in the display are used more than other, spatially
differentiated aging may result, causing portions of the display to
be dimmer than other portions when driven with a similar
signal.
[0009] OLED devices can employ a variety of light-emitting organic
materials patterned over a substrate that emit light of a variety
of different frequencies, for example red, green, and blue, to
create a full-color display. Referring to FIG. 4, a graph
illustrating the typical light output of an OLED display device as
current is passed through the OLEDs is shown. The three curves
represent typical performance of the different light emitters
emitting differently colored light (e.g. red, green and blue light
emitters, respectively) as represented by luminance output over
time or cumulative current. As can be seen by the curves, the decay
in luminance between the differently colored light emitters can be
different. The differences can be due to different aging
characteristics of materials used in the differently colored light
emitters, or due to different usages of the differently colored
light emitters. Hence, in conventional use, with no aging
correction, the display will become less bright and the color, in
particular the white point, of the display will shift. Moreover,
patterned deposition is difficult, requiring, for example,
expensive metal masks. Alternatively, it is known to employ a
combination of emitters, or an unpatterned broad-band emitter, to
emit white light together with patterned color filters, for example
red, green, and blue, to create a full-color display. The color
filters may be located on the substrate, for a bottom-emitter, or
on the cover, for a top-emitter. For example, U.S. Pat. No.
6,392,340 entitled "Color Display Apparatus Having
Electroluminescence Elements" issued May 21, 2002 illustrates such
a device. Such designs are useful because they do not suffer from
differential color aging although they are still susceptible to
differential aging due to different usage of different areas in the
OLED display. In particular, this occurs when the screen displays a
single graphic element in one location for a long period time. Such
graphic elements can include stripes or rectangles with background
information, for example such as news headlines and sports scores,
network logos, and the like. Differences in signal format are also
problematic.
[0010] All televisions utilize a display device to transform video
information into light. This is typically accomplished through the
use of electronic controls that convert the video information into
control signals that operate the display device. However, display
devices may vary in their size, resolution, and aspect ratio, among
other characteristics. Likewise, the video information format,
resolution, and aspect ratio may vary. Hence, the video information
provided to a television may not correspond to the characteristics
of the display device used in the television or other display
device. In particular, the aspect ratio of the video information
may not match the aspect ratio of the display.
[0011] This problem typically arises when video signals formatted
with one aspect ratio are displayed on a television with a display
device having a different aspect ratio. The aspect ratio of a
television picture image is a ratio of horizontal length to
vertical length, expressed in relative units. Standard video
signals, such as NTSC and PAL video signals, are formatted with a
4:3 aspect ratio (i.e., 1.33 aspect ratio), whereas non-standard
video signals, such as HDTV video signals, are formatted with an
aspect ratio greater than the standard 4:3 aspect ratio. For
example, an HDTV video signal is typically formatted with a 16:9
aspect ratio (i.e., 1.77 aspect ratio). Modern cinematographic
theater movies, not made expressly for conventional television, are
typically films with aspect ratios greater than 1.33, typically
ranging between 1.65 and 2.35.
[0012] When standard video signals are displayed on a standard
television screen (i.e., a television screen having a 1.33 aspect
ratio), the picture image appears on the entire television screen.
As long as these standard video signals are displayed on a standard
television screen, the display device is illuminated over the
entire viewing area of the display. When a non-standard video
signal, such as an HDTV video signal, is displayed on a standard
television screen, a region of the display that would normally be
illuminated in response to a standard video signal is not
illuminated in response to the non-standard video signal. As a
result, e.g., the picture image appears on the middle horizontal
region of the television screen and black (non-illuminated) bars
appear on the respective top and bottom horizontal regions of the
television screen. Likewise, television screens having a 16:9
aspect ratio will illuminate the central portion of the display and
have black bars on either side of the display in response to a
standard video signal.
[0013] For example, referring to FIG. 5, a first display device 100
has a first screen aspect ratio of 4:3 and displays a video signal
having the same aspect ratio to illuminate a region 105 comprising
the entire display area of the first display device 100. In this
case, the external video signal is suited to the display device.
Referring to FIG. 6, a second display device 102 has a second
screen aspect ratio 16:9 and displays an external video signal
having the same aspect ratio to illuminate a region 107 comprising
the entire display area of the second display device 102. Again, in
this second case, the external video signal is suited to the second
display device. However, referring to FIG. 7, in a third case if
the first display device 100 receives a video signal having a
different aspect ratio of 16:9, regions 104 of the display are not
illuminated while region 105 is illuminated. Similarly, referring
to FIG. 8, in a fourth case if the second display device 102
receives an external video signal having a different aspect ratio
of 4:3, regions 106 of the display are not illuminated while the
region 107 is illuminated.
[0014] For some display devices, illumination of one portion of a
display device only does not have an effect on the display device.
For example liquid crystal devices use a backlight to illuminate
the entire viewing area of the display even if only a portion of
the display has information. In this case, the light illuminating
the region of the display that has no information is blocked by the
liquid crystals. For other display devices such as OLEDs or plasma
displays, however, illuminating one region of a display and not
others for any significant period of time results in differential
aging such that the illuminated area is aged and the dark areas are
not. When a standard video signal is then displayed on a standard
television screen on which non-standard video signals have been
displayed over an extended period of time, the top and bottom
horizontal regions of the television screen as illustrated in FIG.
7 will be distinctly brighter than the middle horizontal region of
the television screen. A similar phenomenon occurs when a standard
video signal is displayed on a non-standard television screen for
an extended period of time (as illustrated in FIG. 5), causing the
middle vertical region of the nominal scanning area of the display
appear darker than the respective left and right vertical regions
of the display. This differential aging phenomenon results in
visible artifacts when the display is uniformly illuminated. Most
viewers will complain about this phenomenon.
[0015] This problem has been addressed for televisions using a
cathode ray tube display. U.S. Pat. No. 6,359,398 B1 entitled
"Method to control CRT phosphor aging" issued 20020319 describes
methods and apparatus that are provided for equally aging a cathode
ray tube (CRT). A video input terminal is coupled to the CRT and
receives an external video signal. Control circuitry is provided,
which detects the aspect ratio of the signal and determines whether
there is a mismatch between the signal aspect ratio and an aspect
ratio of a display screen in association with the CRT. If a
mismatch between the signal aspect ratio and the screen aspect
ratio exists, an equalization video signal is derived from the
external video signal. A primary region of the CRT is illuminated
in response to the external video signal, and a secondary region of
the CRT, which would otherwise be unilluminated in response to the
external video signal due to the mismatch between the signal aspect
ratio and the screen aspect ratio, is illuminated in response to
the equalization video signal. In this manner, the CRT is uniformly
aged. However, the solution proposed requires the use of blocking
means such as doors or covers that may be manually or automatically
provided to shield the non-illuminated areas from view when the
equalization video signal is applied to the otherwise
non-illuminated region of the display. This solution is unlikely to
be acceptable to most viewers because of the cost and
inconvenience. U.S. Pat. No. 6,359,398 also discloses that
secondary regions may be illuminated with gray video having
luminance intensity matched to an estimate of the average luminous
intensity of the program video displayed in the primary region. As
indicated therein, however, such estimation is not perfect,
resulting in a reduced, but still present, non-uniform aging. U.S.
Pat. No. 6,369,851 entitled "Method and Apparatus to Minimize Burn
Lines in a Display" issued 20020409 describes a method and
apparatus for displaying a video signal using an edge modification
signal to reduce spatial frequency and minimize edge burn lines,
and/or a border modification signal to increase brightness of image
content in a border area of a displayed image, where the border
area corresponds to a non-image area when displaying images with a
different aspect ratio. However, these solutions may cause
objectionable image artifacts, for example reduced sharpness or
visibly brighter border areas in displayed images.
[0016] The general problem of regional brightness differences due
to icon burn-in of specific areas due to video content has been
addressed in the prior art, for example by U.S. Pat. No. 6,856,328
B2 entitled, "System and method of displaying images" Logos may be
present in images transmitted by television stations. These logos
are often present in the corners of an image for a long time. They
do not move and may comprise saturated colors. This results in
burn-in effects in emissive displays because the logos provide the
same display load at the same location for a relatively long period
of time. The burn-in effect can be prevented by detecting the logos
in the corners of the image and reducing their intensity to the
average display load. This method requires the detection of static
areas and may not prevent color-differentiated burn-in. An
alternative technique is described in JP2005037843 A entitled
"Camera and Display Control Device". In this disclosure, a digital
camera is provided with an organic EL display that is prevented
from sticking (burning in) by employing a DSP in the digital
camera. The DSP changes the position of an icon on the organic EL
display by changing the position of the icon image data in a memory
every time that the camera is started. Since the degree to which
the display position is changed is approximately one pixel a user
cannot recognize the change in the display position. However, this
approach requires a prior knowledge and control of the image signal
and does not address the problem of format differences.
[0017] US2005/0204313 describes a further method for display screen
burn prevention, wherein an image is gradually moved in an oblique
direction in a specified display mode. Similarly, commercial plasma
television products advertise pixel orbiter operational modes that
sequentially shifts the image three pixels in four directions
according to a user-adjustable timer. However, these techniques may
not employ all pixels of a display, and therefore may create a
border effect of pixels that are brighter than those pixels in the
image area that are always used to display image data.
[0018] Accordingly, there is a need for an improved method and
apparatus for reducing non-uniformities in a display device.
SUMMARY OF THE INVENTION
[0019] In accordance with one embodiment, the invention is directed
towards a method of driving an elelctroluminescent (EL) display
having a plurality of light-emitting display elements having
outputs that change with time or use, comprising the steps of:
[0020] a) providing an external image signal with a first image
aspect ratio;
[0021] b) providing an EL display having light-emitting display
elements formed in a two-dimensional array having a second display
aspect ratio different from the first image aspect ratio;
[0022] c) driving all of the two-dimensional array of display
elements with a composite signal comprising the external image
signal and an internal aging signal, wherein a subset of the
display elements is driven by the external image signal and the
remainder of the display elements that are not driven by the
external image signal are driven with the internal aging signal;
and
[0023] d) changing the location of the subset of display elements
within the two-dimensional array driven by the external image
signal over time.
ADVANTAGES
[0024] The advantages of this invention include providing an EL
display device that reduces differential aging of the EL materials
and the TFTs in the display when displaying images with aspect
ratios not matched to the display without requiring extensive or
complex circuitry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a flow diagram of a method according to one
embodiment of the present invention;
[0026] FIG. 2 is a schematic diagram illustrating a display useful
in carrying out the method of the present invention;
[0027] FIG. 3A is a schematic diagram illustrating an array of
light-emitting elements and various high-definition format subsets
of light-emitting elements according to various embodiments of the
present invention;
[0028] FIG. 3B is a schematic diagram illustrating an array of
light-emitting elements and various standard-definition format
subsets of light-emitting elements according to various embodiments
of the present invention;
[0029] FIG. 4 illustrates the lifetime of typical OLED
materials;
[0030] FIGS. 5-9 are diagrams illustrating groups of light-emitting
elements in various standard and high-definition formats.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Referring to FIG. 1, a method of driving an EL display
having a plurality of light-emitting display elements having
outputs that change with time or use, comprises the steps of
providing 200 an external image signal with a first image aspect
ratio; providing 202 an EL display having light-emitting display
elements formed in a two-dimensional array having a second display
aspect ratio different from the first image aspect ratio; driving
204 all of the two-dimensional array of display elements with a
composite signal comprising the external image signal and an
internal aging signal, wherein a subset of the display elements is
driven by the external image signal and the remainder of the
display elements that are not driven by the external image signal
are driven with the internal aging signal; and changing 220 the
location of the subset of display elements within the
two-dimensional array driven by the external image signal over
time. The composite signal may be formed 212 in a variety of ways
using techniques known in the prior art, for example by digitizing
206 and storing 208 the external image signal in a memory having a
size of the two-dimensional array of display elements at the
desired address locations in the memory, forming 210 an internal
aging signal by writing a signal value into the memory address
locations that do not store the digitized external image signal
values, and reading out 214 the entire memory through a digital to
analog convertor to the display. Other means for forming the
composite signal may employ analog techniques relying on switching
between the external image signal and the internal aging signal to
write the external image signal and the internal aging signal to
the appropriate locations on the EL display.
[0032] Referring to FIG. 2, in accordance with a further embodiment
of the invention, an OLED display 10 includes a plurality of
light-emitting display elements 12 having outputs that change with
time or use. Controller 16 receives an external image signal 18
with an image aspect ratio different from the display 10 aspect
ratio, and drives all of the two-dimensional array of display
elements 12 with a composite signal 20 comprising the external
image signal 18 and an internal aging signal 19. A subset of the
display elements 12 is driven by the external image signal 18 and
the remainder of the display elements 12 that are not driven by the
external image signal 18 are driven with the internal aging signal
19. Controller 16 also changes the location of the subset of
display elements within the two-dimensional array driven by the
external image signal 18 over time. The change in subset location
may preferably be very slow, possibly once per hour or even once
per operating cycle (i.e. each time the display is powered up) so
that the change in subset is indiscernible to a viewer. The change
in location of the subset may be as large as perceptually
acceptable, within the boundaries of the two-dimensional array of
display elements. Minimally, the change in location may be as small
as a shift in one dimension or another, or both, by a single
pixel.
[0033] Because the aspect ratio of the display and the external
image signal are different and, according to the present invention
not all of the display elements 12 are driven by the external image
signal 18 at the same time, the number of display elements 12 in
the subset of the display elements is smaller than the total number
of display elements 12 in at least one dimension of the
two-dimensional array of display elements 12. In a further
embodiment of the present invention, the number of display elements
12 in the subset of the display elements may be smaller than the
total number of display elements 12 in each dimension of the
two-dimensional array of display elements 12. Moreover, the number
of light-emitting display elements 12 in the two-dimensional array
may be larger than the number of image elements in each
corresponding dimension of the external image signal 18.
[0034] The present invention is particularly useful for EL display
devices that display both high-definition television or standard
definition television format signal and the aspect ratios may
correspond to the aspect ratios of these standards. A standard
definition television format signal has an aspect ratio of 4:3
whereas a high-definition television format signal has an aspect
ratio of 16:9. Referring to FIG. 3A, a subset 30 of an array of
light-emitting display elements 12 in display 10 has a border 40a,
40b of three light-emitting element in both dimensions. In this
embodiment, the subset 30 of light-emitting display elements 12
specifies a high-definition format (16:9). At a given time, the
subset 30 of light-emitting display elements 12 may display the
external image signal 18. All other light-emitting elements 12 that
are not within subset 30, i.e. those light-emitting elements 12 in
the borders 40a and 40b, are driven with the internal aging signal
19. Over time, the controller 16 changes the location of the subset
30 to a different location, for example 30', and drives the
light-emitting display elements 12 in the subset 30' with the
external image signal 18. Once again, the remaining light-emitting
display elements 12 that are not within subset 30' are driven with
the internal aging signal 19.
[0035] Referring to FIG. 3B, the display 10 of FIG. 3A is shown
with a subset 32 of light-emitting display elements 12 specifying a
standard-definition format (4:3). At a given time, the subset 32 of
light-emitting display elements 12 may display the external image
signal 18. All other light-emitting display elements 12 that are
not within subset 32, i.e. those light-emitting display elements 12
in the borders 42a and 42b, are driven with the internal aging
signal 19. Note that in this case the borders 42a and 42b are not
of equal size. Because the display 10 is of approximately
high-definition aspect ratio and the external image signal 18 is a
standard-definition signal, the borders 42b on either side are
wider than the borders 42a on the top and bottom. Over time, the
controller 16 changes the location of the subset 32 to a different
location, for example 32', and drives the light-emitting display
elements 12 in the subset 32' with the external image signal 18.
Once again, the remaining light-emitting display elements 12 that
are not within the subset 32' are driven with the internal aging
signal 19. In this embodiment of the present invention, it may be
preferred to set limits on the locations of the subsets 32, 32'.
Because the border 42b on either side are much greater than the
borders 42a above and below, a user may perceive that the external
image signal 18 is not symmetrically located in the center of the
display 10. Hence, limits 44a and 44b may be applied to limit the
subset locations to subset groups that fall between the limits.
[0036] According to the present invention, the light-emitting
display elements 12 have outputs that change with time or use. If
any light-emitting display elements 12 display a brighter or dimmer
signal, for example logos, headlines, box scores, etc., for a
lengthy period of time, the light-emitting display element 12 will
be darker or brighter compared to neighboring light-emitting
elements when they are all illuminated with a signal specifying a
common brightness. Hence, the light-emitting elements that are
driven by the external display signal 18 will become less bright
over time. According to the present invention, the light-emitting
display elements 12 that are not driven by the external image
signal 18 are driven by an internal aging signal 19.
[0037] Internal aging signal 19 may, e.g., be set to drive the
remaining light-emitting display elements (i.e., those that are not
driven by the external image signal 18) at the average luminance of
the external image signal 18. While such embodiment may provide the
minimum differential aging, Applicants have determined through
experimentation that simply driving the internal aging signal 19 at
the average of the external image signal 18 provides an
unacceptable dynamic variation in the display borders that may be
very distracting. Employing the expected average brightness of a
series of external image signal scenes over time may reduce the
dynamic variation, but still leaves room for inaccuracies based on
the presumed average brightness. Hence, an alternative internal
aging signal value may be desired.
[0038] Applicants have also determined that, in some circumstances,
a border that is darker than the average luminance of the external
image signal 18 may be desired, particularly when a displayed scene
content is darker than the average luminance of displayed scene
contents over time. Hence, in various embodiments of the present
invention, the internal aging signal may have a luminance value of
90% or less of the average luminance value of the external image
signal over time, and more preferably 75% or less of the average
luminance value of the external image signal over time. At the same
time, it is necessary that the internal aging signal have
sufficient brightness to effectively age the border pixels and
reduce differential aging. Hence, in various embodiments of the
present invention, the internal aging signal also may have a
luminance value of at least 10% of the average luminance value of
the external image signal over time, and more preferably at least
25% of the average luminance value of the external image signal
over time. In a more specific embodiment, the internal aging signal
may have a luminance value approximately 50% of the average
luminance value of the external image signal over time.
[0039] Because reducing the internal aging signal to a value less
than the average external signal value over time will result in
less aging of the border pixels, absent countervailing measures a
difference in brightness may eventually be perceived between the
subset and border pixels. According to the present invention,
however, the location of the external image signal values in the
display is changed over time. This change will reduce the spatial
frequency of any differential aging effect. Since the human eye is
less sensitive to low-frequency changes in luminance than to
high-frequency changes in luminance, a more pleasing viewing
environment is provided. The magnitude of the changes in location
will determine the extent of the spatial frequency reduction of the
artifacts resulting from differential aging. Note that, unlike some
other prior-art proposals, this technique does not reduce the
spatial frequency of the scene content itself.
[0040] In some embodiments of the present invention, the internal
aging signal 19 may be dependent upon a portion of the external
image signal, for example the internal aging signal may be limited
to the average luminance value of a portion of the external image
signal, changing dynamically as the external image signal changes
or averaged over time. The portion may correspond to edges of the
display that are formed by display elements driven by the internal
aging signal rather than the external image signal. In yet another
embodiment, the internal aging signal may combine elements of
various signals, for example the internal aging signal may be
limited to driving light-emitting elements 12 to the smaller of the
average scene luminance over time and the minimum luminance of a
border of the external image signal scenes. By combining these
limitations, a more acceptable user viewing experience and a higher
image quality may be obtained. In an alternative embodiment of the
present invention, the internal aging signal may depend upon the
relative amount of time that the light-emitting elements are driven
with the external image signal or on the average luminance of the
external image signal over time. Since display elements that are
driven for some of the time with the external image signal and some
of the time with an internal aging signal may age at different
rates, the aging signal may be adapted to consider the relative
amount of time display elements are driven by the different
signals.
[0041] Referring to FIG. 9, some video recordings are intended for
playback on a display having a different aspect ratio than the
source video content. In such recordings, a border 100 may be added
to the edges of the video content 107 to form a reformatted video
image signal having a format different than the content. Since a
dark border is typically employed, pixels displaying the border
area 100 will not be aged at the same rate as those displaying the
central portion of the video content 107 of the reformatted video
image signal. Where the present invention is employed to display
such a reformatted video image signal in a subset of display
elements on a display having an aspect ratio different from that of
the reformatted video image signal, the display controller may
sample the video content to determine if it has been reformatted.
If so, the border areas of the reformatted video signal may be
likewise driven with an internal aging signal to match the area 106
of the display elements not driven by the reformatted video
signal.
[0042] It is not necessary that all of the light-emitting display
elements 12 driven by the internal aging signal 19 be illuminated
identically. In other embodiments of the present invention,
light-emitting display elements 12 driven by the internal aging
signal 19 adjacent to the light-emitting elements driven by the
external image signal 18 may be driven at a level lower than those
farther away, to increase the contrast at the edge of the external
display image scene. Alternatively, the light-emitting display
elements 12 driven by the internal aging signal 19 adjacent to the
light-emitting elements driven by the external image signal 18 may
be driven at a level that combines the external image signal 18
adjacent to the border values with some other value, for example
the average or minimum scene value. In this way, the differential
aging effect caused by any structures at the edge of a scene may be
mitigated. For example, the light-emitting display elements 12 in
the border and adjacent to the light-emitting elements 12 driven by
the external image signal 18 may be driven to a level that is an
interpolated value of the adjacent light-emitting display elements
12 driven by the external image signal 18 and an average
brightness. In this embodiment, the internal aging signal depends
on the average brightness of a portion of the external image signal
over time. In an alternative embodiment, the display elements 12
adjacent to a border may have a reduced luminance so that the
transition from display elements that are driven by the external
image signal to display elements driven by a less-bright internal
aging signal is more gradual and visually appealing.
[0043] In an alternative embodiment of the present invention, the
internal aging signal may be set to correspond to a fraction of the
maximum display luminance. Since displays are typically calibrated
to emit light at a particular maximum luminance level, this
calibration setting can also be used to determine the internal
aging signal luminance. This approach obviates the need to measure
or consider the brightness of an external image signal.
[0044] External research on television scene content has
established that that average luminance of broadcast scenes over
time may be no more than 12% of the maximum brightness of a series
of scenes. Other estimates range up to 18% or 20%. Hence, the
border light-emitting elements may be driven to those levels to
provide an aging effect comparable to the aging effect of external
image signal 18. In such embodiments, the internal aging signal may
be dependent on the relative amount of time that the light-emitting
elements are driven with the external signal or the relative
brightness of the external signal or both. Applicants have created
a display having border light-emitting elements and tested the
border light-emitting elements and determined that the use of such
border light-emitting elements is acceptable under a wide range of
illuminations and scene contents.
[0045] In this test, applicants have determined that a level of 5%
of the maximum brightness of a series of scenes provides useful
burn-in prevention and is acceptable to viewers. Moreover, a level
of up to 20% is also acceptable, particularly in bright ambient
illumination conditions and for applications requiring a brighter
display. However, since most content is not that bright, on
average, a level of between 10% and 15% may be optimal for some
applications. Moreover, the brightness may be dependent on the
average ambient illumination incident on the display, thereby
employing greater accommodation under conditions where user
objections may be reduced. For example, the internal aging signal
luminance may be increased as the average ambient illumination
incident on the display increases.
[0046] By moving the location of the subset within the display 10,
the edges separating areas having different light-output efficiency
will become blurred, making them much less visible. Lower frequency
structures within a scene are much less visible than
higher-frequency structures. As illustrated in the Figures, the
external image signal 18 may be desirably located in the
approximate center of the display to provide a pleasing viewing
experience. Hence, the location changes of the subset of
light-emitting elements driven by the external display signal 18 in
the array of light-emitting display elements 12 over time may be
greater in one dimension than the other. For example, in the
configuration of FIG. 3B, a standard-definition format signal is
displayed on an HDTV display and the location changes of the subset
is greater in the horizontal dimension than in the vertical
dimension. Since the borders 42b on either side of the subset 32
are much larger than the borders 42a above and below the subset 32,
there is plenty of room to move the subset a greater distance,
thereby reducing the frequency of any vertical structures in the
scene that result from unequal usage and providing a pleasing,
centered viewing experience. In particular, since most television
displays sold today employ a high-definition 16:9 aspect ratio, the
configuration of FIG. 3B is frequently found and the borders on the
sides would be unequally aged due to presenting different format
content. Hence, moving the subset horizontally may be very helpful
in mitigating the format difference problem. Hence, the external
image signal may have an aspect ratio that is relatively larger
than the display aspect ratio, and the location changes of the
subset is greater in the vertical dimension than in the horizontal
dimension. Alternatively, the external image signal has an aspect
ratio that is relatively smaller than the display aspect ratio, and
the location changes of the subset is greater in the horizontal
dimension than in the vertical dimension.
[0047] Ideally, for maximum effect in reducing differential aging,
the external image signal 18 would be moved all the way to either
vertical edge on a high-definition receiver; however, to maintain
an approximately centered image, viewers may not prefer to see the
signal displayed all the way to the edges and an internal aging
signal on only one side of the external image signal (hence the use
of limits 44). Alternatively, a high-definition format signal on a
standard-definition display may employ greater movement in the
vertical dimension (not shown). In alternative embodiments,
external image signal may be displayed to the edges of the display
and cropped in one dimension.
[0048] In a further embodiment of the present invention, the
light-emitting elements of the display form pixels, each pixel
emitting four different colors of light and having four sub-pixels
that each emit one of red, green, blue, and a broadband light. Such
a display may be known as an RGBW display. In this case, a variety
of signals may be employed to drive the four sub-pixels to achieve
a similar color and brightness. However, in order to age such
pixels on the borders of the display, it is necessary to drive each
of the sub-pixel colors at a level corresponding to the relative
average usage of the sub-pixels, rather than the average color or
brightness of the pixel itself. This is defined by the controller
and typically depends on the rendering algorithm employed to
convert an RGB color signal to an RGBW signal. In one case, the W
(broadband) pixel is driven to the maximum neutral brightness
possible (a white mixing ratio of one). In this case, the W
sub-pixel is effectively driven to the neutral luminance signal and
the remaining color sub-pixels driven to the color signal. In an
alternative case, only some of the neutral luminance signal is
represented by the W sub-pixel, and the remainder of the signal
stays with the color sub-pixels. For example, to minimize
differential aging, if a gray color is the average pixel color, the
gray color should not be rendered by driving the white sub-pixel
alone, but rather with a combination of all four sub-pixels. In any
case, the sub-pixels in the border area should be driven at a level
corresponding to their relative use prescribed by the rendering
algorithm.
[0049] In a preferred embodiment of the present invention, the
sub-pixels preferably age at a common rate with respect to the
other border pixels and also with those pixels in the non-border
area. This may be accomplished, for example, by driving the
sub-pixels at a common current density. Alternatively, if the
materials in the sub-pixels age at different rates in response to
current passing through the materials, different currents may be
employed for materials having different aging rates. The currents
may be chosen so that the broadband light-emitting sub-pixel and
the red, green, and blue sub-pixels have a common lifetime.
[0050] It is known to employ amorphous silicon driving circuitry
with each sub-pixel. Since the amorphous silicon materials also
experience aging in the form of changes in their threshold voltage
response, the present invention may be employed to reduce such
performance differences in the transistors employed in the display
elements subset and border areas of a display. By controlling and
equalizing the total current passing through the sub-pixel
circuitry, such performance differences may be reduced.
[0051] The present invention can be constructed simply, requiring
only a conventional display controller with some additional
circuitry for creating an internal aging signal and combining it
with an existing input signal. Such circuitry is well known in the
art and may comprise, for example, an analog-to-digital convertor
(for analog input signals), a frame store, logic for writing data
into the frame store and for reading data from the frame store and
processing it, a clock, and some non-volatile memory.
[0052] Testing completed by applicant has demonstrated that
flat-panel devices are typically hotter in the center than at the
edges. Since OLED materials tend to age faster in the presence of
heat, it may be preferred to over compensate pixels in the borders
to age the border pixels at a rate comparable to the typically
hotter pixels driven by the external image signal. The extent of
over-compensation may be dynamically controlled by measuring the
temperature of the display and matching the compensation to the
temperature measurement.
[0053] In a preferred embodiment, the invention is employed in a
device that includes Organic Light-emitting Diodes (OLEDs) which
are composed of small molecule or polymeric OLEDs as disclosed in
but not limited to U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to
Tang et al., and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to
VanSlyke et al. In another preferred embodiment, the present
invention is employed in a flat panel inorganic LED device
containing quantum dots as disclosed in, but not limited to U.S.
Patent Application Publication No, 2007/0057263 entitled "Quantum
dot light emitting layer" and pending U.S. application Ser. No.
11/683,479, by Kahen, which are both hereby incorporated by
reference in their entirety. Many combinations and variations of
organic, inorganic and hybrid light-emitting displays can be used
to fabricate such a device, including both active- and
passive-matrix LED displays having either a top- or bottom-emitter
architecture. In other embodiments, the present invention is
employed in plasma display devices.
[0054] The present invention can be employed in most EL device
configurations. These include very simple structures comprising a
single anode and cathode to more complex devices, such as passive
matrix displays comprised of orthogonal arrays of anodes and
cathodes to form light-emitting elements, and active-matrix
displays where each light-emitting element is controlled
independently, for example, with thin film transistors (TFTs). It
may be employed in both top- and bottom-emitter configurations.
[0055] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
PARTS LIST
[0056] 10 EL display [0057] 12 light-emitting elements [0058] 16
controller [0059] 18 external image signal [0060] 19 internal
display aging signal [0061] 20 composite image signal [0062] 30,
30' 16:9 aspect ratio subset of light-emitting elements [0063] 32,
32' 4:3 aspect ratio subset of light-emitting elements [0064] 40a,
40b border of light-emitting elements [0065] 42a, 42b border of
light-emitting elements [0066] 44a, 44b limit [0067] 100 display
[0068] 102 display [0069] 104 region [0070] 105 region [0071] 106
region [0072] 107 region [0073] 200 provide external image signal
step [0074] 202 provide display step [0075] 204 drive display with
composite signal step [0076] 206 digitize external image signal
step [0077] 208 store digitized external image signal step [0078]
210 form internal aging signal step [0079] 212 form composite
signal step [0080] 214 read composite signal step [0081] 220 change
location step
* * * * *